Insulating cone plasma enhancement configuration for a plasma waveguide switch

ABSTRACT

A cone made of insulating material is used for plasma enhancement in a plasma waveguide switch (PWS). The long insulating surface yields an ideal potential distribution for injection of the plasma into the waveguide.

United States Patent 1 [111 3,845,427

Schubert Oct. 29, 1974 [54] INSULATING CONE PLASMA 3,480,828 11/1969 Goldie 333/13 ENHANCEMENT ONF A O FOR A 3,509,410 4/1970 Banas et a1. 313/205 PLASMA WAVEGUIDE SWITCH Inventor: David C. Schubert, Pasadena, Md.

Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, D.C.

Filed: Nov. 14, 1973 Appl. No.: 415,893

US. Cl 333/98 S, 313/205, 315/39 R Int. Cl. H01p 1/14 Field of Search 333/98 S, 99 MP, 13;

References Cited UNITED STATES PATENTS 5/1949 Yando 315/39 Primary Examiner.lames W. Lawrence Assistant Examiner-Wm. H. Punter Attorney, Agent, or FirmHarry A. Herbert, Jr.; George Fine [57] ABSTRACT 4 Claims, 1 Drawing Figure TQANODE SIDE OF PULSE FORMING NETWORK TO CATHODE SIDE OF PULSE FORMING NETWORK TO TRIGGER PMENIEMN 29 @914 TO ANODE SIDE OF PULSE FORMING NETWORK L... GENERATOR NO.2

TO TRIGGER 4- GENERATOR No.1

HEATER 4- 230 SOURCE 22b TO CATHODE' SIDE OF PULSE FORMING NETWORK CROSS REFERENCE TO RELATED PATENTS The present invention is related to the patent application entitled, PLASMA WAVEGUIDE SWITCH 1 PERMI'ITING TRIGGERING WITH BOTH CATH- ODE AND WAVEGUIDE GROUNDED, by David C. Schubert, Ser. No. 415,894, filed at an even date herewith.

BACKGROUND OF THE INVENTION Gaseous plasmas are used to switch RF energy in a waveguide by reflecting the RF wave. At low frequencies, the plasma may be generated by a simple thyratron-like arrangement in which a pierced waveguide section serves as the thyratron grid. At frequencies of 60 GHz and above, a focusing cone is employed to compress the plasma and thus obtain the required electron density without imposing undue current density requirements upon the cathode. The cathode-cones waveguide-anode system may be regarded as a gas tetrode. Prior to firing the anode is normally held positive with respect to the other parts by means of a charged pulse forming network. Either the cathode or the grid may be grounded. Firing is accomplished by starting a local discharge between cathode and cone by means of a suitable trigger pulse.

For this discharge to spread through the waveguide and fire the anode pulse forming network (PFN), it is necessary for some plasma to drift to the apex of the cone where it may be influenced by field lines which originated at the anode. Such plasma drift depends upon field lines which originate at the waveguide slot structure and pass through the apex of the cone. These lines spread rapidly to the equipotential cone surface, thus producing a field which falls off rapidly with dis tance from the apex of the cone. Thus the first plasma to be generated at the base of the cone is not pulled very quickly toward the waveguide. This condition tends to produce an undesirable delay in initiating of the main discharge after the trigger pulse is applied. It is also possible that the difficulty in maintaining potential may account for generating of the discharge at high currents.

A configuration which provides considerably more accelerating field is called the gradient cone. The trigger pulse is divided through a suitable resistor network so that during triggering the segments from cone base to apex are at successively more positive potentials. When this configuration was first built it was believed that the continuously maintained acceleating field would permit triggering of a PWS with both the cathodeand the waveguide grounded. This hope was not realized, but it was shown that the gradient cone provided triggering with smaller pulses and less delay between the start of the pulse and the main discharge. Weighed against this advantage was the far greater fabrication problem associated with a four-segment cone requiring four external leads. A configuration which provided the trigger characteristics of the gradient con'e without requiring so many external leads would be a worthwhile advance.

In the present invention .there is utilized a cone made of insulating material for plasma enhancement in the aforementioned plasma waveguide switch in place of the one-piece conducting cone or the four-segment gradient cone which have been previously used. The long insulating surface yields an ideal potential distribution for injection of the plasma into the waveguide. If used with ring electrodes at each end of the cone, the insulating cone will yield pulse characteristics at least as good as the gradient cone although the saving of two external leads greatly simplify construction.

SUMMARY OF THE INVENTION An insulating cone of plasma enhancement configuration is provided which combines the advantages of the solid cone and the gradient cone. This use of an insulating cone exposed directly to the electron beam of a plasma waveguide switch is the unique improvement over existing plasma enhancement techniques. The plasma of the waveguide switch is generated by a simple thyratronlike arrangement in which a pierced waveguide section serves as the thyratron grid. The insulating cone comprises the plasma to obtain the required electron density without imposing undue current density requirements upon the cathode. The cathodeconeswaveguide-anode system may be regarded as a gas tetrode. Prior to firing the anode is normally held positive with respect tothe other parts. Either the cathode or the grid may be grounded. Firing is accomplished by starting a local discharge between cathode and cone by means of a suitable trigger pulse.

DESCRIPTION OF THE DRAWINGS The single figure of the invention shows an insulating cone plasma waveguide switch with electrodes at each end of the cone. Curved lines with arrows show principal electric fields at trigger time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Now referring to the single FIGURE, there is illustrated the insulating cone plasma waveguide switch. The conventional plasma waveguide switch is similar to the present plasma waveguide switch with the exception of insulating cone 10 with electrodes 11 and 12 at each end of the cone. It is noted insulated parts are shown with cross hatching.

There is shown waveguide 13 having slots 14a 1412 which provide a pierced waveguide section. Thus there is formed a thyratron type grid. Waveguide 13 includes flanges 13a and 13b. Waveguide 13 has disposed at one end at flange 13a pressure window 16 to permit the input of an RF signal and disposed at the other end at flange 13b is pressure window 17 for the outputting of the RF signal. Pressure windows 16 and 17 also serve as output seals for waveguide 13.

Housings l8 and 19 are attached to and integrated with waveguide 13 and also are airtight sealed thereto. They serve as insulating envelopes. In housing 18 there is positioned anode 20which is connected to anode lead 21. Anode lead 21 is airtight sealed with housing a 18. Housing 19 has disposed therein heater 22 having airtight sealed leads 22a and 22b. Also included in housing 19 is cathode 23 having airtight sealed lead 23a. Insulating cone 10 is also positioned in housing 19 and connected thereto are electrodes 11 and 12 which are airtight sealed to housing 19. The airtight plasma waveguide switch is comprised of waveguide 13, housings l8 and 19 and the associated elements therein.

The airtight plasma waveguide switch permits a low pressurecontrolled atmosphere operation therein. The aforesaid atmosphere is suitable for supporting a glow discharge. Slots or holes 14a 14h in the walls of waveguide 13 permit establishment of an externally initiated discharge with high currents passing from cathode 23 on one side of waveguide 13 to anode 20 on the other side. A high density plasma is thus established inside waveguide 13 by suitable excitation of anode 20 and cathode 23. Cone 10 is employed to compress the plasma to obtain the required electron density. Radio frequency waves directed into one end of waveguide 13 by way of pressure window 16 are reflected back to their source when plasma of sufficient density fills waveguide 13, but they pass through waveguide 13 virtually unimpeded when plasma is absent.

Insulating cone 10 may be made of ceramic, glass, or other high vacuum composition. Cone 10 provides beam confinement and density plasma enhancement. This use of an insulating wire exposed directly to the electron beam is one of the unique improvements over existing plasma enhancement techniques.

To obtain optimum firing characteristics, two electrodes 11 and 12 are used at the ends of cone 10. Without any external leads to focusing cone l and waveguide 13 grounded, the discharge could be started by applying a negative pulse to cathode 23. It is more reasonable to use a trigger electrode located at the apex of cone to draw plasma that far with the cathode grounded. Electrode 11 is positioned at the base of cone l0 and electrode 12 at the apex thereof. Second electrode 12 at the base of cone 10 is not strictly necessary to start the discharge but permits use of low pulse voltages for triggering. Alternately, it could be used to maintain a local discharge through use of a DC keepalive supply.

The combination of two electrodes 11 and 12 at opposite ends of the cone is uniquely favorable for establishment of a continuous potential gradient from the region just positive of the cathode fall to the waveguide entrance. This continuous gradient, certainly smoother than that provided by the gradient cone, assures that the plasma will rush into the waveguide with minimum elapsed time after the start of the trigger pulse. After firing of anode 20, insulating cone l0 maintains bound ary conditions for a positive column plasma which is less subject to quenching because of the potential gradient which is naturally maintained. Both the solid conducting cone and the gradient cone tend, on the other hand, to degrade this required potential gradient by imposing a unipotential boundary condition. Thus the present invention provides conditions for triggering at low trigger voltages and currents to at least the performance level of the gradient cone and with the simplicity of only two instead of four external leads. At somewhat reduced efficiency, this invention can be embodied with only one external lead as in the solid conducting cone, but still with better field distributions than are present in the solid conducting cone. Conventional gas tube practice calls for scrupulous avoidance of insulating materials in the electron beam path. The present invention is an important exception to that rule.

What is claimed is:

l. A plasma waveguide switch for broadband control of RF energy transmission comprised of a section of waveguide having opposite walls, each of said walls having a set of slots in the center portion thereof to provide first and second sets of slots, said section of wave guide having first and second ends, first and second pressure windows airtight sealed to said first and second ends for entrance and exit of RF energy, respectively, a first insulating envelope airtight sealed to one of said opposite walls fully encompassing said first set of slots, a second insulating envelope airtight sealed to the other of said opposite walls fully encompassing said second set of slots, an anode in said first insulating envelope physically separated from and adjacent to said first set of slots, said anode being connected to a first external lead, a cathode positioned in said second insulating envelope, said cathode being connected to a second external lead, an insulating cone having a base and apex with the base adjacent to and physically separated from said cathode and said apex separated from and adjacent to said second set of slots, a first trigger electrode connected to said base, said first trigger electrode having a third external lead, a second trigger electrode connected to said apex, said second electrode having a fourth external lead, said section of waveguide, first and second envelopes being airtight sealed to each other forming a housing for said plasma waveguide switch, and a low pressure controlled atmosphere in said housing for supporting a glow discharge.

2. A plasma waveguide switch as described in claim 1 wherein said insulating cone is comprised of ceramic.

vacuum nonconduction composition material. 

1. A plasma waveguide switch for broadband control of RF energy transmission comprised of a section of waveguide having opposite walls, each of said walls having a set of slots in the center portion thereof to provide first and second sets of slots, said section of waveguide having first and second ends, first and second pressure windows airtight sealed to said first and second ends for entrance and exit of RF energy, respectively, a first insulating envelope airtight sealed to one of said opposite walls fully encompassing said first set of slots, a second insulating envelope airtight sealed to the other of said opposite walls fully encompassing said second set of slots, an anode in said first insulating envelope physically separated from and adjacent to said first set of slots, said anode being connected to a first external lead, a cathode positioned in said second insulating envelope, said cathode being connected to a second external lead, an insulating cone having a base and apex with the base adjacent to and physically separated from said cathode and said apex separated from and adjacent to said second set of slots, a first trigger electrode connected to said base, said first trigger electrode having a third external lead, a second trigger electrode connected to said apex, said second electrode having a fourth external lead, said section of waveguide, first and second envelopes being airtight sealed to each other foRming a housing for said plasma waveguide switch, and a low pressure controlled atmosphere in said housing for supporting a glow discharge.
 2. A plasma waveguide switch as described in claim 1 wherein said insulating cone is comprised of ceramic.
 3. A plasma waveguide switch as described in claim 1 wherein said insulating cone is comprised of glass.
 4. A plasma waveguide switch as described in claim 1 wherein said insulating cone is comprised of a high vacuum nonconduction composition material. 